This project is designed to study the pharmacokinetics of the distribution and elimination of several narcotics in the ovine maternal-fetal unit following acute single dose administration and with continuous infusion to steady state. Findings to date include: 1. Fetal renal clearance of both meperidine and methadone are greater than inulin clearance indicating renal tubular secretion of both drugs. 2. Calculations of the half-life of both drugs from plasma decay curves and urinary excretion rates yield similar estimates indicating that the rate of drug appearance in the fetal urine is directly proportional to the amount of drug in the fetus. 3. Fetal plasma-brain equilibration time for both drugs is dependent on the route of administration. Following i.v. administration equilibration occurs rapidly and at a high level whereas it occurs later at a lower level following i.m. administration. 4. At steady state the plasma concentrations of both drugs are lower in the fetal than in the maternal compartment after correction for protein binding and pH indicating fetal elimination via metabolism and renal excretion.